The invention relates to an apparatus for measuring such as, for example, a coordinate measuring apparatus or a machine tool which includes a control and evaluation unit and a measuring sensor. The measuring sensor operates independently of said control and evaluation unit and can be displaced with a mechanism along three coordinate directions relative to a workpiece, which is to be measured. The invention further relates to a method for measuring using such an apparatus.
Such an apparatus for measuring can, for example, be a coordinate measuring apparatus or a machine tool. The sensors used in such apparatuses are most often optical probes such as, for example, triangulation probes or video cameras, which include evaluation electronics. For a video camera used as a measuring sensor, the system clock rate of the evaluation electronics is defined by the video frequency of the camera. This frequency is significantly greater than the system clock rate of the control unit of a coordinate measuring system or a machine tool, which is optimized to the drive control. A synchronization of the measuring sensor with the control and evaluation unit without additional special measures can only be achieved if an evaluation is made at standstill of the coordinate measuring apparatus or machine tool, since only then the values of the measuring sensor and the machine measuring values of the coordinate measuring apparatus do not vary in time. However, performing a big measuring task in this way requires a tremendous amount of time, since for the recordation of each measuring point the coordinate measuring apparatus has to come completely to standstill.
To overcome this problem, complicated interfaces have been developed between the control and evaluation unit of a coordinate measuring apparatus or machine tool and a measuring sensor functioning independently of such control and evaluation unit.
Such coordinate measuring apparatus is described in U.S. Pat. No. 5,982,491, which is incorporated herein by reference. This United States patent discloses a laser triangulation probe mechanism which functions independently from the coordinate measuring apparatus and can be moved by the mechanics of a coordinate measuring apparatus along the coordinate directions x, y, z for detecting the edges on workpieces. For this purpose, the measuring sensor is guiding a probe beam on a circular scanning line and the coordinate measuring apparatus mechanism guides the measuring sensor perpendicularly over the edge. To synchronize the sensor values of the measuring sensor, which have been evaluated, with the machine measuring values of the scales of the coordinate measuring apparatus, both the coordinate measuring apparatus and the measuring sensor include an interface, which is especially developed for the measuring sensor used. Via this interface, the measuring sensor informs the control unit of the coordinate measuring apparatus on how many electronic cycles ago the measuring sensor was in a defined position with respect to the edge which has been traversed.
The object of the present invention is to provide an apparatus for measuring having a control and evaluation unit and a measuring sensor, which functions independently from the control and evaluation unit but which can be easily synchronized therewith. It is another object of the invention to provide a method for measuring using such an apparatus.
The apparatus of the invention is for measuring a workpiece and includes: a control and evaluation unit generating machine measuring values (xi, yi, zi) which includes a first timer; a measuring sensor detecting sensor measuring values (xxe2x80x2i, yxe2x80x2i, zxe2x80x2i) and including a second timer; the first and second timers operating independently of each other; the measuring sensor being coupled with the control and evaluation unit; a mechanism operatively connected to the control and evaluation unit for displacing the measuring sensor in three coordinate directions relative to the workpiece in response to the measuring values (xxe2x80x2i, yxe2x80x2i, zxe2x80x2i); and, a synchronizing device for synchronizing the first timer and the second timer.
The method of the invention is for measuring a workpiece using an apparatus including a control and evaluation unit coupled with a measuring sensor and an electromechanical drive mechanism for displacing the measuring sensor in three coordinate directions in relation to the workpiece. The control and evaluation unit includes a first timer and the measuring sensor includes a second timer, the first and the second timer operating independently of each other, the method including the steps of: generating machine measuring values (xi, yi, zi) with the control and evaluation unit; generating the sensor measuring values (xxe2x80x2i, yxe2x80x2i, zxe2x80x2i) with the measuring sensor; and, synchronizing the first timer and the second timer.
The basic concept of this invention is to provide timers in the measuring sensor and in the control and evaluation unit which function independently of each other. The timers are synchronized to a common start time point.
The important advantage of such a coordinate measuring apparatus or machine tool over the prior art is that such coordinate measuring apparatus allows to synchronize in a simple way measuring sensors, which function independently of the coordinate measuring apparatus, with the control and evaluation unit of the coordinate measuring apparatus or the machine tool. In this way, a very simple standard can be provided, which can be taken over by the manufacturers of the corresponding measuring sensors without any big effort. Furthermore, it is also possible to update in a very simple way old measuring sensors, which do not have this standard, just by adding a small program section. This standard provides decisive advantages also for the manufacturers of coordinate measuring apparatuses and machine tools. On the one hand, the often time and cost intensive adaptation of a coordinate measuring apparatus or a machine tool to a specific measuring sensor type hereby is unnecessary. On the other hand, the variety of sensors usable on a coordinate measuring apparatus or a machine tool can be increased considerably.
As a timer in the measuring sensor and in the control and evaluation unit, the timer present in the system having the highest resolution should be used. As an example, this means for a personal computer, the clock counter register for the cycle interrupt source should be used or, for a microprocessor, the time stamp counter register should be used. This is necessary because relatively small time increments are required. The system clock of a personal computer, for example, which provides only for 18 time increments per second, is not sufficiently accurate for this purpose.
The synchronization of the timers to a common starting point can be done in different ways. Most simply, the time values of the timers are read out at a certain time and stored as the start time point. In a preferred embodiment, the timers are synchronized to values, which represent a common clock time. Preferably, the timers should be set to world time as is the case in so-called radio clocks.
To make the running deviations between the timers as small as possible, the timers should be synchronized several times during a measuring sequence at short time intervals to a common start time point.
The time relation of the time values outputted by the timers relative to each other can be determined in a very simple embodiment, in that, at a certain time point after the start point, the common time values of the timers are read out again and the difference of these time values to the corresponding time values at the start time point is determined. The quotient of these differences indicates the ratio of the run time of the timers relative to each other.
Preferably, the timers are normalized to a common time unit such as, for example,sa second. In this way, the time values, which originate from the timer of the measuring sensor can be directly compared to the time values, which originate from the timer of the control and evaluation unit. For normalization, a plurality of different variations is possible. The simplest variation is that the control and evaluation unit as well as the measuring sensor normalize the corresponding timers themselves. This is possible in a simple manner in that the number of cycles of the corresponding timer are counted until the corresponding system clock has advanced by a defined time interval, such as, for example, one second. The time increment per cycle of the timer then results from the time interval divided by the number of cycles counted.
As an alternative, it is also possible, that such normalization is performed by the control and evaluation unit. For example, this could be achieved in that the control and evaluation unit outputs a signal via a trigger line, based on which both the timer in the measuring sensor and the timer in the control and evaluation unit are read out. Then, after the system clock of the control and evaluation unit has advanced by a certain time interval, another signal is outputted via the trigger line and in correspondence to this signal, the timer in the measuring sensor as well as the timer in the control and evaluation unit are read out. The time increment per cycle of the corresponding timer then results in the same way as the time interval divided by the number of cycles counted.
So that the control and evaluation unit can reasonably cooperate with the measuring sensor, the detected measuring values of the measuring sensor are tagged with a time stamp by its timer and transmitted to the control and evaluation unit. Since the control and evaluation unit is synchronized with the measuring sensor via the timers, the sensor measuring values can be adapted in time to the measuring process in the control and evaluation unit at any time.
For example, if the measuring sensor values shall be fed back to the travel path data of the measuring process, it is necessary to adapt the sensor measuring values to the measuring process. For example, this is the case if the surface of a workpiece is to be scanned continuously with a laser triangulation probe. For this purpose, the measuring sensor is guided by the control of the coordinate measuring apparatus over the surface of the workpiece parallel to the surface, whereby the distance of the measuring sensor from the surface is held constant in that the measuring sensor values are fed back to the travel path data so that the mechanism for the measuring sensor corrects the position of the measuring sensor in the corresponding direction.
For this feedback, a dead time is calculated for each sensor measuring value, which results from the time difference between the time stamp of the sensor measuring value and the actual time value of the timer in the control unit. This dead time is of interest for two reasons.
On the one hand, long dead times show that the data transmission line between the measuring sensor and the control and evaluation unit or the measuring sensor itself is working relatively slowly. For this reason, the control and evaluation unit should be designed in a way that the measuring speed of the measuring sequence is reduced the longer the dead time becomes.
As described above, the reason for the feedback is also to maintain a constant distance of the measuring sensor from the surface of the workpiece. The older the measuring sensor value, the smaller the effect on the control correction should be.
For this reason, the control variable should be calculated in a way that, in case the dead time is increasing, the effect of feedback is reduced.
Furthermore, when the sensor measuring values are mathematically processed with the machine measuring values of the control and evaluation unit, the sensor measuring values also have to be adapted to the measuring sequence. That means, the sensor measuring values must be processed with the scale values of the x, y and z direction of the mechanism. For this purpose, the sensor measuring values have to be brought into relationship with the machine measuring values.
In order to achieve this, time stamps are added to the machine measuring values, which were provided by the timer in the control and evaluation unit, so that the time stamps of the sensor measuring values can be compared to the time stamps of the machine measuring values.
By interpolating between sensor measuring values or between machine measuring values, pairs of sensor measuring values and machine measuring values can be determined, which correspond to each other and which then can be processed for determining measuring points.
Furthermore, the sensor measuring values with the time stamps can also be used for correcting the measuring results. Since the time stamps subsequently define the exact time sequence of the sensor measuring values, this time sequence of sensor measuring values can be analyzed and as an example an analysis of oscillations can be made. For example, the sensor measuring values can be Fourier transformed to determine the characteristic oscillations of the coordinate measuring apparatus.
In general, as a coordinate measuring apparatus, any coordinate measuring apparatus can be used, which includes a measuring sensor that can be displaced in the three coordinate directions of the measuring system. Such coordinate measuring apparatus can be a stand measuring apparatus, a portal measuring apparatus, a bridge measuring apparatus or a coordinate measuring apparatus with axes of rotation. The coordinate measuring apparatus can have a numerically controlled drive or can be based on manual control.
In the same way, also a broad variety of machine tools is possible. For example, such machine tool could be a milling machine, which includes a measuring sensor instead of a milling tool.
The same holds for the measuring sensors, which can be of various types. For example, a laser triangulation probe could be used as a measuring sensor or a video camera or an interferometric sensor.